This invention relates to a method of producing an improved titanium-containing catalyst and its use in an epoxidation process. The catalyst is obtained by reacting a tert-alkyl trihydroxysilane with a titanium complex. The method is a simple procedure to form soluble catalysts from commercially available reagents. The catalyst is highly active for olefin epoxidation.
Many different methods for the preparation of epoxides have been developed. Generally, epoxides are formed by the reaction of an olefin with an oxidizing agent in the presence of a catalyst. The production of propylene oxide from propylene and an organic hydroperoxide oxidizing agent, such as ethyl benzene hydroperoxide or tert-butyl hydroperoxide, is commercially practiced technology. This process is performed in the presence of a solubilized molybdenum catalyst, see U.S. Pat. No. 3,351,635, or a heterogeneous titania on silica catalyst, see U.S. Pat. No. 4,367,342.
Catalyst improvements are desired for this commercially practiced technology. U.S. Pat. No. 5,750,741 discloses the use of soluble titanasilsesquioxanes in the epoxidation of olefins. The homogeneous titanasilsesquioxane catalysts are taught as highly active and selective catalysts when compared to solubilized molybdenum. PCT Intl. Appl. No. WO 99/49972 also discloses that these titanasilsesquioxanes are useful as activators for deactivated heterogeneous titania on silica catalysts.
One potential disadvantage of these new homogeneous catalysts is the difficulty in preparing the titanasilsesquioxanes. The catalysts require the initial preparation of an incompletely condensed silsesquioxane [R7Si7O9(OH)3] which is described in U.S. Pat. No. 5,750,741 as taking from greater than five days, when R is cyclopentyl, to greater than six months, when R is cyclohexyl. Feher, et al., Organometallics (1991) 10, 2526, also describe the synthesis of incompletely condensed silsesquioxane in low yield taking from more than seven days, when R=cyclopentyl, to greater than six weeks, when R=cycloheptyl.
Due to the difficulty in forming the incompletely condensed precursor, a new method for forming active homogeneous titanium catalysts would be worthwhile. Winkhofer, et al., Angew. Chem. Int. Ed. Eng. (1994) 33, 1352, describe the formation of titanasilsesquioxanes having a 1:1 Si:Ti ratio from stabilized silanetriols. Winkhofer also describes that for the reaction of silanetriol t-BuSi(OH)3, it proved advantageous to use SnMe3 protecting groups.
In sum, new olefin epoxidation catalysts and new processes to form them are needed. Particularly valuable would be processes that form high activity and high selectivity catalysts in simple, timely steps.
The invention is a method of producing an olefin epoxidation catalyst. The method comprises reacting a trihydroxysilane having the chemical formula R1R2R3Cxe2x80x94Si(OH)3 with a titanium complex having the chemical formula TiLn, wherein R1, R2, and R3 are the same or different C1-C10 hydrocarbyl, n is 3 or 4, and L is halide, alkoxide, 2,4-alkanedionate, silyloxide, amide, cyclopentadienyl, hydrocarbyl, or mixtures thereof. The ratio of the R1R2R3Cxe2x80x94Si(OH)3:TiLn is seven.
We surprisingly found that the new method produces catalysts that are active and selective in olefin epoxidation and are made in a much simpler process than previous prior art methods.
The method of the invention comprises reacting a trihydroxysilane, substituted with a tertiary alkyl group, with a titanium complex so that the Si:Ti ratio is 7:1. It is surprisingly found that the tertiary alkyl substituent is necessary in order to produce catalysts with high activity and selectivity in olefin epoxidation compared to secondary and linear alkyls (see Table 1). The Si:Ti ratio is also unexpectedly important in forming active and selective catalysts (see Table 2).
The trihydroxysilane has the chemical formula R1R2R3Cxe2x80x94Si(OH)3, wherein R1, R2, and R3 are the same or different C1-C10 hydrocarbyl group. Preferably, R1, R2, and R3 are the same or different C1-C4 alkyl or a C6-C10 aryl group. Particularly preferred trihydroxysilanes include t-butyl trihydroxysilane and t-amyl trihydroxysilane.
The titanium complex has the formula TiLn, wherein n is 3 or 4 and L is halide, alkoxide, 2,4-alkanedionate, silyloxide, amide, cyclopentadienyl, hydrocarbyl, or mixtures thereof. Preferably, the titanium complexes include chloride, bromide, iodide, ethoxide, isopropoxide, acetylacetonate (2,4-pentanedionate), trimethylsiloxide, dimethyl amide, diethyl amide, cyclopentadienyl, or benzyl substituents. Most preferably, the titanium complex is titanium(IV) isopropoxide, titanium(IV) chloride, or tetrabenzyltitanium.
The reaction of trihydroxysilane and titanium complex is preferably performed in a solvent. Suitable solvents include any known hydrocarbons, oxygenated hydrocarbons, and halogenated hydrocarbons. Examples of suitable solvents include toluene, n-hexane, n-heptane, cyclopentane, ethanol, isopropanol, diethyl ether, acetone, methylene chloride, chloroform, chlorobenzene, and the like.
The product of the reaction is typically a solid compound. The product is preferably purified by any suitable purification method before use. Preferably, the product is purified by crystallization. Crystallization methods are well known in the art.
The trihydroxysilane is prepared by any suitable method. Preferably, the trihydroxysilane is formed by the hydrolysis of a trihalosilane. The trihalosilane compound has the chemical formula R1R 2R3Cxe2x80x94SiX3, wherein R1, R2, and R3 are the same or different C1-C10 hydrocarbyl group and X is a halide. Preferred halides include chlorine and bromine. In the hydrolysis reaction, the trihalosilane is reacted with water, preferably in a H2O:R1R2R3Cxe2x80x94SiX3 ratio of 3:1 to allow formation of the trihydroxysilane with no excess water. Because hydrohalic acid, HX, is formed during the hydrolysis of trihalosilane, a basic compound is typically added to the reaction mixture. The basic compound reacts with HX to form a stable base.HX salt that is readily separated from the trihydroxysilane product. Although any basic compound that forms a stable salt with HX is suitable, preferred basic compounds include aliphatic and aromatic amines, particularly ethylamine, diethylamine, triethylamine, pyridine, aniline, piperidine, and the like.
The epoxidation process of the invention comprises contacting an olefin with an organic hydroperoxide in the presence of the catalyst of the invention. Suitable olefins include any olefin having at least one carbon-carbon double bond, and generally from 2 to 60 carbon atoms. Preferably the olefin is an acyclic alkene of from 3 to 10 carbon atoms such as propylene, butene, pentene, hexene, heptene, octene, nonene, decene, and isomers thereof. Also preferred are olefinically unsaturated compounds substituted with a hydroxyl group or a halogen group such as allyl chloride or allyl alcohol. Particularly preferred olefin is propylene.
Preferred organic hydroperoxides are hydrocarbon hydroperoxides having from 3 to 20 carbon atoms. Particularly preferred are secondary and tertiary hydroperoxides of from 3 to 15 carbon atoms, especially secondary alkyl hydroperoxides wherein the hydroperoxy group is on a carbon atom attached directly to an aromatic ring, e.g., ethylbenzene hydroperoxide. Other exemplary organic hydroperoxides suitable for use include t-butyl hydroperoxide, t-amyl hydroperoxide, cyclohexyl hydroperoxide, and cumene hydroperoxide.
In such an epoxidation process the olefin:hydroperoxide molar ratio is not particularly critical, but it is preferable to employ a molar ratio of from 1:1 up to 20:1.
The epoxidation reaction is conducted in the liquid phase in solvents or diluents that are liquid at the reaction temperature and pressure and are substantially inert to the reactants and the products produced therefrom. In commercial practice, it will generally be most economical to use as a solvent the hydrocarbon used to produce the organic hydroperoxide reactant. For example, when ethylbenzene hydroperoxide is utilized, the use of ethylbenzene as the epoxidation solvent is preferred. It is conducted at moderate temperatures and pressures. Typically, the organic hydroperoxide is present at concentrations of from about 1 to 50 percent by weight of the epoxidation reaction mixture (including olefin). Suitable reaction temperatures vary from 0xc2x0 C. to 200xc2x0 C., but preferably from 25xc2x0 C. to 150xc2x0 C. The reaction is preferably conducted at or above atmospheric pressure. The precise pressure is not critical. The reaction mixture may, for example, be maintained substantially in a non-gaseous phase or as a two-phase (gas/liquid) system. The catalyst formed by the reaction of trihydroxysilane and titanium complex is soluble and thus is present in the liquid phase during the epoxidation process of this invention. Typical pressures vary from 1 atmosphere to 100 atmospheres.
The epoxidation may be performed using any of the conventional reactor configurations known in the art for reacting olefin and organic hydroperoxide in the presence of a soluble catalyst. Continuous as well as batch procedures may be used. The reaction solvent, the catalyst, and any unreacted olefin or organic hydroperoxide may be recycled for further utilization.